The present invention relates to defibrillators and, in particular, to portable defibrillators used under emergency circumstances to apply external defibrillation or therapeutic pulses to heart attack victims or patients suffering from ventricular fibrillation. More specifically, the present invention relates to defibrillators designed to detect potentially dangerous conditions within the defibrillator, such failures of certain components within the defibrillator that may cause a safety hazard to the operator or patient, and which disable the application of defibrillation pulses to a patient in the event such conditions exist.
Portable defibrillators have been in existence for a substantial period of time. One example of such a prior art defibrillator is disclosed in U.S. Pat. No. 4,823,796 to Benson, incorporated in its entirety by reference herein. Although damped sinusoidal and other defibrillation wave forms may be used in defibrillators, including the defibrillator of the present invention, the Benson patent discloses a defibrillator in which a trapezoidal shaped defibrillation pulse is applied to a patient. In accordance with the Benson patent, an electronic pulse delivery switch is in a series path which includes the patient and charge storage capacitors charged by a capacitor charging circuit. A single pole, double throw switch relay is also included in the Benson series circuit together with the electronic switch. Upon closing of the relay switches in response to a transfer relay signal, and following the rendering of the electronic switch conducting by a switch control signal, a discharge path is provided through the patient for discharging the charge storage capacitors. In the Benson patent, the electronic switch is rendered nonconducting to terminate the defibrillation pulse after the desired energy has been delivered to the patient. In this case, the resulting pulse is generally trapezoidal in shape. An energy selection circuit is provided for use in selecting the amount of energy to be applied to the patient by the defibrillation pulse. In addition, the Benson device has a voltage detection circuit provided for measuring the charge on the charge storage capacitors for use in determining when the capacitors are charged to the desired level for delivery of a defibrillation pulse of the selected energy.
The Benson defibrillator circuit has been incorporated into a commercially available defibrillator sold under the trademark Heartstart 1000 by Laerdal Medical Corp. of Armonk, N.Y., U.S.A. This particular defibrillator has a single microprocessor for controlling the performance of the defibrillator. In a conventional manner, the Heartstart 1000 defibrillator, and another commercially available defibrillator sold by Laerdal Medical Corp., the Heartstart 2000, monitors ECG signals from a patient and analyzes these signals according to a protocol. The purpose of this analysis is to determine when a patient is in cardiac distress of the type for which the application of a defibrillation pulse is appropriate, such as ventricular fibrillation and ventricular tachycardia. In addition to the protocol used in the Heartstart 1000 defibrillator, a number of protocols for analyzing ECG signals for shockable conditions are known, such as disclosed in U.S. Pat. No. 4,432,375 to Angel, et al. and in a report entitled "Standard for Automatic External Defibrillators," dated Nov. 14, 1989, from the Association for the Advancement of Medical Instrumentation of Arlington, Va.
The Heartstart 1000 and 2000 defibrillators are each operable in several modes, including a semi-automatic mode. In a semi-automatic mode, upon the determination of a shockable cardiac distress condition in a patient, a shock indicating message is visually displayed, and a shock indicating auditory message is provided to the operator of the defibrillator. In response to these signals, the operator may manually actuate a shock to initiate the delivery of a defibrillation pulse to a patient. The Heartstart 2000 defibrillator is also operable in a manual mode with the operator of the device simply monitoring the patient's condition without using the analysis protocol. A shock may then be initiated upon the determination that a shockable condition exists. In addition, automatic defibrillators are also known, including the Heartstart 1000 defibrillator, wherein ECG signals are automatically analyzed and a shock is applied without the intervention of the operator of the defibrillator, in the event a shockable cardiac distress is determined as a result of the analysis.
The Heartstart 2000 defibrillator, as well as other known defibrillators, typically monitor the impedance across the patient. If the impedance is outside of a desired range, an indication is provided that, for example, the patient coupling electrodes are incorrectly positioned or coupled to a patient.
The Heartstart 2000 defibrillator, as well as other defibrillators, have been used on many occasions to deliver defibrillation pulses to patients and bring the patients out of life threatening cardiac distress. However, for additional safety and enhanced performance, improvements in these devices are desirable. For example, in these devices there is a possibility of a failure of internal components which may result in the potential for delivering a defibrillation pulse to a patient in circumstances where the defibrillation pulse may not be appropriate.
The publication entitled "Microcomputers in Safety Technique, an Aid to Orientation for Developer and Manufacturer," published by Verlag Tuv Rheinland Gmblt, Koln in 1984, is a document relating to the use of microcomputers in safety-related applications, including medical electrical equipment, which would include defibrillators. The basic philosophy of this report is that no single fault which is assumed to arise will lead to a dangerous failure in the apparatus. Although providing general guidelines relating to safety design approaches, this particular article does not disclose specific designs for defibrillators. However, some of the general criteria set forth in this article are described below. In connection with software and restarts, for instance as a result of a "reset" situation, the article mentions the requirement of passing through a safe state. In addition, monitoring of the supply voltage of a system is mentioned in the article along with the taking of proper actions (e.g. going into a safe condition, switching off a processor, or switching to another channel) in the event the voltage falls below the specified limits of components in the system. In connection with two-channel structures, the article mentions the approach of comparing the results of complementary tests. In single channel structures, the article refers to the use of high level tests for monitoring ROM, RAM, input/output lines, CPU and also time-based and logic-based program monitoring. The article also makes reference to the use of diversified software. In addition, two-channel structures are described by the article as two independent functional units for carrying out a specified function. The article mentions that the functional units can be identical or that they can be built up in different ways using the principle of diversification (hardware diversification, software diversification, and time diversification). The article mentions that signals which are used or produced by both systems are continuously compared with one another for the purpose of fault detection. In an internal mutual comparison technique, for use in two-channel structures where both channels are implemented as computers, the article mentions that both computers are coupled together by either serial or parallel interfaces and exchange their input data, sometimes intermediate results, and their output data, for purposes of comparison. The comparison is carried out, according to the article, by and within both computers and if any nonequivalences are established by one or both computers, then a transition of the process to a safe state must take place. The computers are described by the article as comparing the signals computed by its partner and also the output signals with the signals computed by itself. In a hardware diversification technique, the results produced by two individual channels are compared with one another, and in the event of any discrepancy, a reaction is initiated for the purpose of a transition to a safe state. Although the Tuv article provides guidance for the development of products in safety-related applications, the article is not understood to suggest specific implementations of these guidelines in defibrillators.
Therefore, a need exists for an improved defibrillator and in particular, for such a defibrillator with reliability and performance verification features.